IEEE Power Electronics Magazine - December 2014 - 14

VIn

IRef
R1

In
+
-

Vcontrol
+

Output
VOut = IRef : R1

AN142 FD1b

IEEE PowEr ElEctronIcs MagazInE

4.8 V-28 V

LT3080-1

Vcontrol
+

1 nF

Corporation, which he cofounded with
Bob Swanson. A few years later, Dobkin's linear team at Linear Technology
created another milestone in this area.
Around 1986 or 1987, the newly founded precision analog company unveiled
high-output-current positive adjustable regulators with very low dropout
and pin compatible with older threeterminal regulators [1]. The LT1083 series was designed to provide up to 7.5 A
with higher efficiency and a maximum
dropout of only 1.5 V at maximum output current, which was substantially
lower than the previous generation and
continued to use only two external resistors to set the output voltage.
During this period, the switching
regulators were encroaching the linear
turf. According to Dobkin, "Switching
regulators began to grow significantly
in the late 1980s and early 1990s, driven by the portable PCs and portable
electronics markets, which required
low voltage and high current." Switching regulators were initially used primarily in offline applications and later
moved to point-of-load applications.
Linear regulator applications were
limited to 5-25 W. "In the 1980s and
1990s, linear regulators were focused
on general-purpose applications. Now,
linear regulators are used when designers want reduced complexity, low
noise, low power, and lower solution
cost," notes Dobkin. The market for
linear regulators has grown as has the
market for switching regulators. "A lot
comes down to a designer's individual
preference," he asserts.
By the early 1990s, the three-terminal linear regulators were going

25 mX Out*

Set

VIn

fig 4 The new architecture replaces
bandgap reference with a current
source and uses a voltage follower for
the output amplifier. (Figure courtesy
of Linear Technology.)

LT3080-1

25 mX Out*

Set
165 k

VOut
3.3 V
2.2 A
10 nF

30801 TA01
fig 5 The internal ballast resistor eases paralleling of LT3080-1 for higher output current. (Figure courtesy of Linear Technology.)

through another major boost. The
n-p-n pass transistor in the output
stage of the design was replaced by
its p-n-p counterpart. The result was
very low dropout and high robustness with low quiescent current and
a ground pin. The low power dissipation also enabled the company
to offer these regulators in surfacemount packages. As a result, using
bipolar technology, Linear introduced the first-generation p-n-p micropower low-dropout linear regulators with adjustable output voltages
in 1992. The first member in this line
was LT1121, with 150-mA output current, adjustable and fixed outputs,
0.4-V dropout, and 30-µA quiescent
current. In addition, it did not require
protection diodes and was available
in multiple package choices. The
company continued to add members
with higher output current capabilities. In 1995, a 3-A version, the LT1529,
was introduced.
The second-generation parts offered lower noise and faster transient
response. Concurrently, the company
also improved the input voltage capability. A good example is the LT3010,
a part that was released in February
2003, and handles a wide input voltage range of 3-80 V. Though LT3010
supported only 50-mA output cur-

z December 2014

rent, a 250-mA version (LT3013) was
added to this line in 2006.

Transition to Current Source
Ever since the introduction of the first
three-terminal adjustable linear regulator in 1976, the architecture has more
or less remained the same. Therefore,
the reference-voltage-dependent adjustable output could not go below
the 1.25-V reference voltage, making
it unsuitable for powering low-voltage
integrated circuits that were emerging
in the market based on low-geometry
complementary metal-oxide-semiconductor processes. A new architecture was needed to end the low-voltage
limitation of three-terminal linear regulators. Linear Technology answered
the call by replacing the bandgap voltage reference with a current source
and using a voltage follower for the
output amplifier [2], [3] (Figure 4). It
was implemented in LT3080, which
was introduced in 2007. Per Dobkin's
explanation in [3], the architecture
delivers two key benefits: the ability to operate down to 0 V and to allow paralleling of regulators for more
output current. Furthermore, because
the output amplifier always operates
at unity gain, both the bandwidth and
the regulation are constant, explains
Dobkin. Also, transient response is

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - December 2014